The present invention relates to a platen for typing and printing machines, particularly matrix printers, wherein a basic support element is provided for transporting a print medium by means of friction.
Printing platens or platen drums are well known, and they are usually provided for air gap free support of paper or the like and they participate in driving and advancing such a print medium in a particular direction which is usually transversely to the direction of printing or, more precisely, of a printing line. The print medium may either be a single sheet or a basically endless web. Also, a single sheet as well as endless web kind of print medium may come in various layers and carbon paper or the equivalent is interposed. Generally this lack in uniformity of the print medium to be accomodated by one and the same printer poses problems because the conditions of friction and therefore the conditions of transport differ.
The platen used for example in ordinary typewriters but also used in some kind of matrix printers is often constructed as a rubber drum or particularly a synthetic drum in order to transport the print medium. Rubber drums as well as platen drums made of synthetic material have to have in either case a high surface friction. In the case of a matrix printer using print elements such as styli i.e. needles or printing hammers having particular points or print balls the situation is present that the force exerted by these impacting print elements is very high so that particularly with increasing speed of printing the hardness of the surface of the platen must be improved as compared with earlier versions. On the other hand it was found that if platen drums are made of a harder-than-rubber material, such as aluminum drums, the friction decreases. In other words increase in hardness has been to some extent compensated by a decrease in friction which then poses problems with regard to the reliability of transporting the print medium.
It can thus be seen that the state of the art is characterized by rubber or synthetic platen drums with relatively high friction for the transport but being limited to lower print speed, and alternatively there are metallic platens having a lower frictional value but being more suitable for higher print speeds because of a larger hardness and resistance against the high speed impact of the print elements.
Generally speaking print speeds have to be increased which is a trend in matrix printer engineering so that a metallic and very hard printing platen will ensure high quality of the characters being printed. But it was found that in such high speed printers the transport of the print medium may sometimes be insufficient amounting to a skewing of the paper. This problem was particularly noticed in case of single sheet feeding. On the other hand particularly in the case of line printers a precisely metered advance of the print medium particularly in the case of high speed printing, is a prerequisite for a good i.e. high quality, easily legible, so called near letter quality appearance of the printed characters.